In the pollen sac (microsporangium) of the anther, haploid microspores are formed by mitosis. Then mitosis occurs to produce a two-celled pollen grain with a small generative cell and a large vegetative cell. This generative cell will undergo further mitosis to form two male gametes. Vegetative cell forms the pollen tube which grows through a pore in the pollen grain, with the tube (vegetative) nucleus at its tips and the male nuclei behind.